1. Field of the Invention
This invention relates to a diaphragm, and, more particularly, to an iris diaphragm for use with optical instruments, such as still cameras and video cameras, for controlling the amount of light.
2. Description of Related Art
Iris diaphragms for use with optical instruments, such as still cameras and video cameras, are often automatically controlled to regulate the amount of light. These diaphragms are divided into two types.
The first type of iris diaphragm has a plurality of sector-shaped diaphragm blades, to one of which a neutral density filter is secured. Although this type of iris diaphragm is adapted to close entirely, its aperture is prevented from excessively decreasing in size when it is automatically controlled. On the other hand, while the camera is not in use, the iris diaphragm is entirely closed to isolate internal elements of the camera from light.
Referring to FIG. 1, showing one of the conventional first type of iris diaphragms, sector-shaped diaphragm blades 10, 12 and 14 are partially lapped one over another so as to form a substantially circular opening, as a diaphragm aperture DP, with their inside edges. The blades are held between a stationary diaphragm ring (not shown) and a diaphragm adjusting ring 24 which are arranged coaxially and are enabled to make relative rotation. These diaphragm blades 10, 12 and 14 are, respectively, pivoted on pivot pins 10A, 12A and 14A secured to the stationary diaphragm ring. Each of the diaphragm blades 10, 12 and 14 is formed with a cam slot 10B, 12B or 14B, adjacent to its pivot pin 10A, 12A or 14A, which is directed approximately radially when the diaphragm blades 10, 12 and 14 are fully opened. The diaphragm adjusting ring 24 is provided with actuator pins 16, 18 and 20 at regular angular separations which are, respectively, received for sliding movement within the cam slots 10B, 12B and 14B of diaphragm blades 10, 12 and 14. Turning of diaphragm adjusting ring 24 causes the diaphragm blades 10, 12 and 14 to turn about pivot pins 10A, 12A or 14A to open and close the iris diaphragm. Diaphragm blade 10, which at the top, as viewed from the stationary diaphragm ring, is provided, at the middle of the inside edge, with a small piece of neutral density filter 22 adhered thereto so as to project within the opening defined by the diaphragm blades 10, 12 and 13.
Turning of the diaphragm adjusting ring 24 in the counterclockwise direction causes diaphragm blades 10, 12 and 14 to turn about pivot pins 10A, 12A and 14A, respectively, in the same direction so as to diminish the diaphragm aperture DP and finally close it entirely. On the other hand, when turning the diaphragm adjusting ring 24 in the clockwise direction, the diaphragm blades 10, 12 and 14 are forced to turn about pivot pins 10A, 12A and 14A, respectively so as to gradually open the diaphragm aperture DP.
If this iris diaphragm is automatically controlled responding to changes in the amount of light, because a distance L1 of a point of action from the pivot pin of diaphragm blade 10, 12, 14 is shorter than a distance L2 of a center of the blade from the pivot pin of diaphragm blade 10, 12, 14, operational plays between the actuator pins 16, 18 and 20 and the cam slots 10B, 12B and 14B of diaphragm blades 10, 12 and 14, respectively, are magnified. Accordingly, the iris diaphragm exhibits a diaphragm aperture of less accuracy. If high accuracy is a requirement, then a feedback control, which in turn will generally slow down a control of the aperture diaphragm, is necessary.
Referring to FIG. 2 showing another type of conventional iris diaphragm, a plurality of circular arcuate-shaped elongated diaphragm blades 30 are lapped one over another so as to form a substantially circular opening, as a diaphragm aperture, with their inside edges and held between stationary diaphragm rings 31. A diaphragm adjusting ring 32 with an adjusting projection 34 is arranged coaxially with and supported for rotation between the stationary diaphragm rings 31. These diaphragm blades 30 are, respectively, each pivoted at one end on pivot pins 30A secured to the stationary diaphragm ring 31 at regular angular intervals. Each of the diaphragm blades 30 is provided at the other end with a pin 30B. The diaphragm adjusting ring 32 is provided with actuator slots 32A at regular angular intervals. The pins 30B of diaphragm blades 30 are, respectively, received in the actuator slots 32A of diaphragm adjusting ring 32.
Turning of the diaphragm adjusting ring 32 in the counterclockwise direction causes diaphragm blades 30 to turn about pivot pins 30A in the same direction so as to open the diaphragm aperture DP. On the other hand, when turning the diaphragm adjusting ring 32 in the clockwise direction, the diaphragm blades 30 are forced to turn about pivot pins 30A so as to diminish the diaphragm aperture DP.
This type of iris diaphragm offers a decrease of operational play caused at a point of action, namely, between the actuator slot 32A and pin 30B, of the diaphragm blade 30. This is because a distance L3 between points of action of the pivot pin of a diaphragm blade 30 is longer than a distance L4 between center of blade 30 and the pivot pin 30A of a diaphragm blade 30. Consequently, the iris diaphragm offers an increase in accuracy of diaphragm aperture.
However, when the iris diaphragm is adjusted to smaller sizes of apertures, the diaphragm blades 30 interfere with one another, so as to render the iris diaphragm difficult to fully close as shown in FIG. 3. Further, it is difficult to attach a neutral density filter to one of the diaphragm blades 30.